Many electric load elements, particularly electric load elements in motor vehicles, such as lamps, heating spirals etc., are driven by means of pulse-width modulation (PWM), wherein the power delivered to the load element can be regulated or controlled, wherein it is possible to minimize the losses in the drive electronics by the switching operations.
However, when driving load elements in motor vehicles by means of pulse-width modulation, electromagnetic fields that may interfere with the radio reception in the vehicle are emitted via the battery supply lines and the load supply lines. Therefore, appropriate limiting values have been laid down in various standards, such as IEC, ISO, CISPR, said limiting values reducing the interference with the radio receiver in the corresponding spectra to a tolerable degree. These emitted fields may be reduced by filtering at the inputs and outputs of the control device, for example. The new methods actively influence the switching edges, as it is described in the unexamined application W02005/057788, for example, which is incorporated by reference.
In the new series of vehicles, the lamps are being increasingly replaced by light emitting diodes (LED). However, the non-linearity of their current-voltage characteristic results in a sudden break-off of the current and so in increased interfering emissions.
Conventional methods attenuate the high-frequency alternating currents in the supply lines by means of filters in the input lines and output lines. However, the disadvantage of the filters consists in the fact that they are very expensive and require a lot of space, whereby they raise the price of the electronic components, and that they cannot be miniaturized (integration in silicon).
Another possibility of reducing electromagnetic radiation is the reduction of the switching speed in the switching element, whereby the high-frequency current portions can be reduced to the necessary degree, but here the undesirable switching losses heating up the electronic components are increasing with decreasing switching speed.
In the new methods, for example according to WO2005/057788, the switching speed of the switching element is varying in dependence on the instantaneous power loss. FIG. 1 shows the normalized power loss in a switching element when driving an ohmic load element (linear load element) as well as a stepped convergence of the course of the rate of change of the output voltage according to WO2005/057788.
However, such methods, as disclosed in said application, fail when driving load elements that show a non-linear behaviour within a switching process (load elements with a non-linear voltage-current interrelationship, such as LEDs).
On account of the non-linear interrelationship between the voltage and the current in the load element, the power loss in the switching element is not linearly dependent on the output voltage or the load current, respectively. Therefore, when driving the non-linear load elements, an adjustment of the switching speed of the switching element that is only related to the power loss or output voltage at the switching element or to a quantity depending thereon is not applicable without being confronted with increased switching losses or a bad electromagnetic radiation, respectively. This difference between linear and non-linear load elements is illustrated in FIG. 5, for example. With an ideal linear load element (e.g. linear resistor), the current changes proportionally to the voltage at the load element (see dashed line L1 in FIG. 6). Thus, with a linear load element, there is a quadric-polynomial interrelationship between the power loss PV at the switching element and the output voltage Ua: PV˜(Ua)2, i.e. the power loss PV is linearly proportional to the square of the output voltage Ua, as shown in FIG. 1 (see continuous polynomial curve).
On the other hand, with a non-linear load element, the current flowing in this non-linear load element does not change proportionally to the output voltage (see continuous line L2 in FIG. 6). Therefore, there is no linear interrelationship between the power loss and the current or the voltage, respectively. This is illustrated in FIGS. 2 and 8, wherein FIG. 2 shows a non-linear interrelationship in the L-range of the output voltage and FIG. 8 a non-linear interrelationship both in the L-range and in the H-range of the output voltage. As a result of the non-linear interrelationship between the voltage and the current, an adjustment of the switching speed only on the basis of the power loss or the output voltage is not practicable with the non-linear load elements, which means that the radiation of electromagnetic noise fields cannot be reduced effectively by means of the method according to WO2005/057788.